This invention relates to die casting and more particularly to an improved method and hot chamber apparatus for pressure die casting.
Die casting is a well known way of shaping articles in which a liquid material, such as molten metal, is placed in a cavity which is formed in the shape of the desired article between separable die members. The liquid material fills the die cavity and solidifies therein in the shape of the desired article. The die members are then separated and the article removed from the die cavity. In pressure die casting, the liquid material is forced or injected into the die cavity under pressure.
Although die casting is a relatively easy way of forming articles, particularly articles having complex exterior surfaces which otherwise would be difficult to form, pores often form throughout the casting to significantly weaken the cast article. Such weak, porous articles are not suitable for many applications and articles for these applications have had to be manufactured by other, more expensive techniques.
Even though die casting and the problems presented by the porosity of cast articles have been long known, it was not until U.S. Pat. No. 3,382,910 issued on May 14, 1968 in the names of Radtke and Eck that a novel way of avoiding pores in cast articles became known. This patent, which is assigned to the same assignee as this application, describes a method of pore-free die casting in which the die cavity is purged of air (or other non-reactive gas or vapor) with a gas which reacts with the material to be cast in the die cavity. Such gas is herein called a reactive gas.
When the material is then cast, the reactive gas in the die cavity reacts with the material to form solid compounds therewith rather than the pores or bubbles which the non-reactive components of air in the die cavity would have formed in the cast article. For example, the patent describes flushing the die cavity with oxygen which reacts with a molten metal as it is cast in the die cavity to form small particles of oxides of the cast metal rather than pores or bubbles of trapped non-reactive gas.
Even though the practice of the method described in U.S. Pat. No. 3,382,910 produced articles which were significantly more pore-free than articles produced by conventional die casting, it has been found that some pores continue to be formed in articles cast according to the patented method in which only the die cavity is purged.
One attempt to control the formation of pores produced in cast articles suggests that portions of the article which first solidified shrank from still liquid portions of the material to form pores in the article. It then proposes to control, but not eliminate, the formation of such pores by controlling the places at which the cast article first cools and solidifies in the die cavity so that the pores from shrinkage of the material are formed in a portion of the cast article which can tolerate weakness from the pores or which may be removed from the finished article.
A proposal for further reducing the pores in an article cast in a die cavity which is filled with a reactive gas suggests placing a constricted gate at the place where the material enters the die cavity. The constricted gate produces a turbulence in the material injected into the die cavity to mix the injected material more completely with the reactive gas in the die cavity. Even if this proposal is more successful in reacting the gas with the material, it alone does nothing to eliminate non-reactive gas which may be injected into the die cavity with the material. Such non-reactive gas in the die cavity then forms pores in the cast article in the same way as if the die cavity had not been purged with the reactive gas before casting the material.
Two types of die casting apparatus are known. One type, often and herein called cold chamber apparatus, has a chamber just large enough to fill the die cavity once. This chamber is generally open to receive the individual shots of material to be cast. The open chamber can be easily flushed with a reactive gas (along with the die cavity) so that non-reactive gas is not forced into the die cavity ahead of the material to form pores in the casting.
The other type of die casting apparatus, often and herein called hot chamber apparatus, however, has a chamber or furnace holding a continuous supply of the material to be cast. This chamber is connected to the die cavity by an enclosed passage for filling the cavity for successive castings. Inasmuch as the passage is enclosed and blocked at one end by the supply of material to be cast, it cannot be readily flushed with reactive gas.
Flushing the die cavity with a reactive gas merely traps non-reactive gas in the passage between the cavity at one end and the material at the other. Filling the die cavity then forces the non-reactive gas into the die cavity where it forms pores in the casting.
Opening the passage so that it could be flushed like cold chamber apparatus cannot be done because the material would then escape through the opening as it fills the die cavity. In cold chamber apparatus, a plunger usually pushes the material away from the opening as it forces the material into the die. In hot chamber apparatus, however, a plunger which pushes the material along the portion of the passage in which non-reactive gas is trapped (a gooseneck, nozzle and sprue bushing, as later described) would not reach the passage to push the material away from an opening for flushing the passage. Indeed, the plunger would push the material toward and out of such an opening. For these and other reasons, the same techniques with which cold chamber apparatus is purged are not suitable for hot chamber die casting apparatus.